1921.] Recent Advances in Stereochemistry. 229 
mechanism of these reactions. The main results are given in 
the following table which shows that in‘all the solvents except 
liquid ammonia and acetonitrile, the sign of the amino acid 
formed is opposite to that of the bromoacid taken. 
Action of Ammonia on |-phenylbromacetic Acid. 
1. 1918, 113, 140. 
Value of [a]'5461 
Solvent. for amino acid formed. 
Liquid NH3 Ye ge — 38° 
Water “a ‘s Le . +89 
HOH «ls; * oy mn +2 
C,H;OH is 4 ae +2 
C3;3H,0H +5°5 
C2H,OH (iso) 
C,H ,OH (n) +17°7 
(CH); C OH 7 
C;H);OH (n) 8 is o's +130 
CHAN.” .. ; —48°3 
he chief resemblances in the two reactions discussed 
above are that in both cases the amino acids obtained from 
bromacetic acid is opposite in sign, from phenylchloracetic 
acid similar in sign, to the original phenylhalogen acid, as 
shown in the subjoined table :— ML OG 
Amino acid from l-bromo acid. 
Solvent (alcohol). Methyl. Ethyl. Propyl. Isopropyl. 
fr +2° +2° +5°5° +3°2 
10 +5 
Amino acid from d-chloroacid. +7'5 +34 
It is thus obvious that Walden’s discovery is a puzzle and 
surprise, for it cannot fit into any pre-existing theory of 
optical activity and it could not have been anticipated as a 
corollary to Van’t Hoff’s theorem of the asymmetric carbon 
atom. It is almost certain that a complete solution of this 
problem will furnish us with the mechanism of the process of 
substitution in chemical reactions. 
have tried to place before you a brief account of some of 
the recent work on optical activity, but till a complete theory 
of dptical activity is forthcoming, the correlation of these 
results cannot be satisfactorily accomplished. Meanwhile the 
duty of the chemist is to accumulate as much experimental 
data under as varied conditions as possible. 
Addendum, 15th July, 1921. 
Prof. J. B. Cohen, F.R.S., of Leeds, in reviewing this 
address in Nature for June 9, 1921, Vol. 107, p. 474, says 
